Studies on the Physiology of Nodule Formation: IV. The Mutual Inhibitory Effects on Nodule Production of Plants grown in Association

The number of root nodules formed on clover in test-tube culture on agar depends directly upon the volume of the medium and inversely upon the number of seedlings present per unit volume of medium. Plants which form nodules early in seedling development inhibit nodule formation on an associated plant more strongly than plants which normally nodulate later. Among all the other types of plants examined, viz. effectively or ineffectively responding plants, sparsely or abundantly nodulating plants, and resistant or normally susceptible plants, it was shown that the mutual inhibiting effects when grown in pairs are everywhere the same in spite of large differences in plant size. Similar inhibitory effects were found in the presence of non-leguminous plants. Successive replanting of the same agar slope also leads to inhibition of infection.

The nodulation of lucerne and vetch is similarly affected by the presence of a companion plant. Nodulation is always reduced by the presence of a plant of the same species, but among the heterogeneous associations, consisting of members of two different cross-inoculation groups, with either one or both of the appropriate strains of nodule bacteria present, inhibition occurs in.some associations and not in others. In such mixed associations inhibition is not related to the numbers of bacteria present or to the pH of medium. The hypothesis is put forward that nodulation may be inhibited by specific substances secreted by the root.
RESP-3164